The United States is adding Mali to the list of countries whose travellers get special Ebola screening after a number of cases in the West African nation, the government said.

Mali is not suffering a widespread Ebola outbreak. But federal officials are growing increasingly alarmed about a new cluster of seven illnesses in Mali that have left public health workers scrambling to track and monitor at least 450 other people who may have had contact with the infected and may be at risk.

"At this point we can't be confident that every exposed person has been identified, or that every identified person is being monitored daily,'' Dr Tom Frieden, director of the US Centers for Disease Control and Prevention, said on Sunday.

Starting Monday, anyone arriving in the US from Mali will undergo the same screening procedures that were ordered last month for travellers from Liberia, Sierra Leone and Guinea. That includes taking arriving travellers' temperatures, and questioning them about their health and possible exposure to the Ebola virus.

They will also be asked to provide contact information and to agree to have daily communications for 21 days with local health officials who will be asking them to take their temperatures twice each day and monitoring them to see if they develop symptoms.

West Africa is currently suffering the worst Ebola outbreak in world history, with at least 14,000 illnesses and more than 5,100 deaths so far.

Nearly all of the cases have been in Guinea, Liberia and Sierra Leone. About 75 people arrive from those three countries each day, on average. They are funnelled through five airports: two in New York and one each in Washington, Chicago and Atlanta.

In contrast, only about 15 to 20 passengers arrive from Mali to the United States on an average day. The majority end up arriving through the same five airports. But in the next few days, steps will be taken to make sure all funnel through those airports, Frieden said.